A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography, plural patterns are projected onto a substrate, e.g. in order to enable manufacturing complex semiconductor structures. These plural patterns are projected consecutively onto the substrate. In order to be able to manufacture patterns at a complexity and small dimensions, a high accuracy of overlay of the patterns is required. In order to reduce so called overlay errors, a plurality of techniques are applied including an alignment of the substrate. In order to align the substrate, alignment measurements are performed by an alignment sensor. The alignment sensor essentially measures a position of one of more known references that are provided on the substrate, the known reference(s) e.g. comprising known patterns such as alignment reference patterns.
In lithography processes, manufacturing cost and its reduction may play a relevant role. As a result, a designer of substrate (e.g. semiconductor) structures aims to obtain a large usable area on a substrate, so as to get as many resulting products from one substrate as possible, thereby to sacrifice a part of the surface of the substrate as small as possible. Alignment marks are generally placed next to usable areas, i.e. next to (e.g. semiconductor structure) patterns on the surface of the substrate, also referred to as target portions. In order to be able to provide a high alignment accuracy and increase a net yield per substrate, a tendency may be observed to provide alignment marks in the patterns on the substrate, e.g. in a lower layer, whereby successive layers on top of the alignment mark provide (e.g. semiconductor) structures. Thus, further layers may be provided on top of the alignment mark, thus using the available substrate surface efficiently. Thereby, a usable space of the substrate surface is increased, and substrate surface that is used for “overhead” purposes only, such as the provision of reference alignment marks, is reduced. Given high overlay requirements, a desire for a large number of alignment marks to be provided on the substrate surface and distributed over the substrate surface, may come into existence. A tendency is observed that the number of layers to be provided onto the substrate tends to increase, causing the number of lithographic patterns to be successively projected onto the substrate, to increase. Due to the fact that a large number of layers may be provided onto the substrate, such alignment mark may be hidden by plural layers provided on top of it during operational use of the lithographic apparatus.